Hi in this video we're gonna be talking about helical formations of D. N. A. So one of the first helical formations that I want to talk to you about. That's really important. And you may not have heard of before is super coiling. So super coiling is a property of helical D. N. A. And so it's kind of hard to imagine. But super coiling is D. N. A. That has twisted upon itself. And so if you can imagine if you have you know two ropes or two pieces of hair or two shoe strings and you make them into a helix. So twist them together when you keep twisting them eventually they fold up on each other. Um And so that's the best way to describe this. And so in D. N. A. Like in ropes or pair shoestrings. Um this also can occur and that can occur in circular or linear D. N. A. But you can imagine this is really bad. Um So if you're twisting a rope and it twisted upon itself, that's not necessarily very a useful rope. Um And then the same with D. N. A. It really prevents a lot of things from happening that should happen with DNA. Like replication or um anything gene expression. And so we need enzymes and things that can break that apart. So these enzymes are called Topo tom racist. It's kind of a mouthful and they are enzymes that convert DNA between super coiled and relaxed state. Now there are two of them. They're type one and type two. And they worked really super coiling by two different mechanisms. So type one does single strand breaks and type two does double strand breaks and they release um breaks. They put breaks into DNA to release tension. So you can see here this is gonna be a super coiled okay D. N. A. And this is a circular D. N. A. And so you can see here that if there's a nick for sort of a break here, there's another one here, it releases this this tension and eventually there's another one here. And so you get this circular DNA molecule which is what it's supposed to look like. And this sort of relaxed stay instead of in a super coiled form. Now there's another way that we can sort of mess with the D. N. A. Helix. And that's through DNA. Featuring which is gonna be separating or re nature ring which is rejoining strands of D. N. A. Now this occurs in cells especially during things like replication or gene expression. But we can also do it in laboratories. And how this happens is to denature D. N. A. It um we can denature it by breaking hydrogen bonds. And so how do we break hydrogen bonds? Well um you sort of break them in ways that you would break other bonds. You can increase the heat, you can change the ph you can expose them to UV light and all of these things will break hydrogen bonds and sort of separate the two DNA strands together. Now in the laboratory we of course need to be able to quantify this. You know how much heat or how much ph and so the way we do heat is we actually say the D. N. A melting temperature. And that's a temperature that separates the D. N. A. Strands. Um But of course like other chemical bonds, this is going to depend on the number of hydrogen bonds present. And so if you remember that the G. C pairs here have an extra hydrogen bonds. So they bond with three hydrogen bonds instead of the A. T. Which used to hydrogen bonds. And so um in strands of DNA that have a really high G. C. Content which means that they have a lot of G. C. Base pairs. That's of course going to raise the temperature needed to break all of those bonds and to separate the strands. So here we have the DNA double helix, you can see that it's separated through here. And um so it's it's the nature ring and eventually things can come back in here and re nature it and which means put it back together, that's what this looks like. So now let's move on
2
Problem
Problem
Which of the following property is false regarding supercoiled DNA?
A
Supercoiling is a helix that has twisted upon itself
B
Supercoiling can be fixed by topoisomerases
C
Supercoiling only happens in circular DNA
D
Supercoiling can happen in both circular and linear DNA
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Problem
Problem
Which enzyme is responsible for repairing supercoiling through double strand breaks?
A
Topoisomerase Type 1
B
Topoisomerase Type 2
C
Topoisomerase Type 3
4
Problem
Problem
What is the name of the temperature that causes two complementary DNA strands to separate?
